Generally, in the process of manufacturing a semiconductor device, fine patterns are formed using photolithography. A number of transfer masks called photomasks are normally used in forming the fine patterns. This transfer mask is generally a fine pattern formed by a metal thin film or the like and provided on a transparent glass substrate, and photolithography is also used in the fabrication of the transfer mask.
A mask blank having a thin film (e.g., a light shielding film (an opaque film)) for forming a transfer pattern (a mask pattern) on a transparent substrate such as a glass substrate is used in fabricating the transfer mask by photolithography. The fabrication of the transfer mask using the mask blank involves a drawing process of drawing a desired pattern on a resist film formed on the mask blank, a developing process of developing the resist film after drawing to form a desired resist pattern, an etching process of etching the thin film as a mask, and a process of stripping and removing the remaining resist pattern with the resist pattern used as a mask. In the developing process, after the desired pattern is drawn on the resist film formed on the mask blank, a developer is supplied to the resist film to dissolve a portion of the resist film which is soluble in the developer, thereby forming the resist pattern. In the etching step, with the resist pattern used as a mask, an exposed portion of the thin film where the resist pattern is not formed is removed by wet etching or dry etching, thereby forming a desired mask pattern on the transparent substrate. The transfer mask is finished in this way.
A phase shift mask, as well as a binary mask having a light shielding pattern of a chromium-based material on a conventional transparent substrate, is known as the types of a transfer mask. The phase shift mask is configured to have a phase shift film on a transparent substrate. The phase shift film has a predetermined phase difference, and is made of, for example, a material containing molybdenum silicide compound. Further, a binary mask using a material containing a silicide compound of a metal such as molybdenum for a light shielding film is being used. These binary mask and phase shift mask are generally called a transmissive mask herein, and a binary mask blank and a phase shift mask blank, which are used as a master for a transfer mask, are generally called a transmissive mask blank herein.
In recent years, higher integration of semiconductor devices in the semiconductor industry requires fine patterns beyond the transfer limit of the conventional photolithography using ultraviolet light. To enable formation of such fine patterns, EUV lithography which is an exposure technique using extreme ultraviolet (hereafter referred to as “EUV”) is promising. EUV light refers to light in the waveband of the soft X-ray region or the vacuum ultraviolet region, specifically, light with a wavelength of about 0.2 to 100 nm. A reflective mask has been proposed as a transfer mask for use in the EUV lithography. Such a reflective mask has a multilayer reflective film formed on a substrate to reflect exposure light, and an absorber film patterned on the multilayer reflective film to absorb exposure light.
The reflective mask is fabricated by forming an absorber film pattern using photolithography from the reflective mask blank including a substrate, a multilayer reflective film formed on the substrate, and an absorber film formed on the multilayer reflective film.
As described above, as a demand for miniaturization in the lithography process increases, problems in the lithography process are becoming prominent. One of the problems concerns defect information on a mask blank substrate and a substrate with a multilayer reflective film, which are used in the lithography process.
The mask blank substrate is demanded to have a higher flatness from the viewpoints of an improvement on the defect quality needed with the recent miniaturization of patterns and the optical characteristics needed for transfer masks. The conventional surface processing methods for mask blank substrates are described in, for example, Patent Literatures 1 to 3.
Patent Literature 1 describes a glass-substrate polishing method of polishing the surface of a glass substrate essentially comprising SiO2 by using a polishing slurry containing colloidal silica with an average primary particle size of 50 nm or less, acid, and water, and adjusted to have a pH of 0.5 to 4, in such a way that the surface roughness, Rms, as measured with an atomic force microscope becomes not more than 0.15 nm.
Patent Literature 2 describes a polishing agent for the synthetic quartz glass substrate, which contains a suppressive colloidal solution and an acidic amino acid to suppress the formation of defects to be detected by a high-sensitivity defect inspection apparatus on the surface of a synthetic quartz glass substrate.
Patent Literature 3 describes a method of controlling the flatness of a quartz glass substrate by placing the quartz glass substrate in a hydrogen radical etching apparatus, and causing hydrogen radicals to act with the quartz glass substrate, so that the flatness can be controlled in the sub-nanometer order.
The substrate with a multilayer reflective film is also demanded to have a higher flatness from the viewpoints of an improvement on the defect quality needed with the recent miniaturization of patterns and the optical characteristics needed for transfer masks. The multilayer reflective film is formed by alternately laminating a high refractive index layer and a low refractive index layer on the surface of the mask blank substrate. Those layers are generally formed by sputtering using sputtering targets made of materials for forming the layers.
As a method of sputtering, ion beam sputtering is preferably carried out, it is because it does not need produce a plasma through discharge, making it difficult to mix an impurity in the multilayer reflective film, and has an independent ion source so that the conditions are set relatively easily. In view of the smoothness and surface uniformity of each layer to be formed, sputter particles are reached the main surface of the mask blank substrate at a large angle to the normal line of the main surface of the mask blank substrate (line perpendicular to the main surface), i.e., at an angle oblique to or nearly parallel to the main surface of the substrate to deposit a high refractive index layer and a low refractive index layer.
As a technique of manufacturing a substrate with a multilayer reflective film with this way, Patent Literature 4 describes that at the time of deposing a multilayer reflective film for a reflective mask blank for EUV lithography on a substrate, ion beam sputtering is carried out by keeping the absolute value of an angle α defined by the normal line of the substrate and the sputtered particles incident to the substrate at 35°≤α≤80° while rotating the substrate about the central axis of the substrate.
Patent Literature 1: JP-A-2006-35413
Patent Literature 2: JP-A-2009-297814
Patent Literature 3: JP-A-2008-94649
Patent Literature 4: Unexamined Japanese Patent Application Publication (Published Japanese Translation of PCT Application) JP-A-2009-510711
With the rapid miniaturization of patterns in lithography using an ArF excimer laser or EUV (Extreme Ultra-Violet), the defect sizes of transmissive masks (also called optical masks), such as a binary mask and a phase shift mask, and an EUV mask that is a reflective mask, are also becoming smaller. To find such fine defects, the wavelength of the inspection light source used in defect inspection is approaching the light source wavelength of the exposure light.
For example, as a defect inspection apparatus for an optical mask, a mask blank, which is a master thereof, and a substrate, high-sensitivity defect inspection apparatuses with an inspection light source wavelength of 193 nm are becoming popular. As an EUV mask, an EUV mask blank, which is a master of an EUV mask, and a substrate, high-sensitivity defect inspection apparatuses with inspection light source wavelengths of 266 nm (for example, the mask substrate/blank defect inspection apparatus “MAGICS M7360” for EUV exposure of Lasertec Corp.), of 193 nm (EUV mask/blank defect inspection apparatus “Teron 600 series” of KLA-Tencor Corp.), and of 13.5 nm are becoming popular.
The main surface of a substrate used for the conventional transfer mask is managed by surface roughness represented by Rms (root mean square roughness) and Rmax (maximum height) in the fabrication process. Because the detection sensitivity of the high-sensitivity defect inspection apparatus described above is high, however, many false defects are detected in defect inspection of the main surface of the substrate even when the smoothness in compliance with Rms and Rmax becomes higher from the viewpoint of the improvement on the defect quality, raising a problem such that the defect inspection cannot be performed to the end.
Further, an attempt is made to deposit the multilayer reflective film of the substrate with a multilayer reflective film used in the conventional transfer mask by, for example, the method described in “Background Art” to reduce recess defects present on the substrate. Even if defects originating from the recess defects on the substrate can be reduced, defect inspection on the multilayer reflective film raises a problem such that many defects are detected (the number of defects detected=critical defects+false defects) because the detection sensitivity of the high-sensitivity defect inspection apparatus described above is high.
The false defect mentioned herein refers to a tolerable irregularity on the substrate surface or the multilayer reflective film, which does not affect pattern transfer, and which is erroneously determined as a defect in inspection with a high-sensitivity defect inspection apparatus. When a lot of such false defects are detected in defect inspection, critical defects that affect pattern transfer may be buried in many false defects, so that the critical defects cannot be discovered. For example, a defect inspection apparatus having an inspection light source wavelength of 266 nm, 193 nm or 13.5 nm, which is currently becoming popular, cannot inspect the presence or absence of critical defects for a substrate or a substrate with a multilayer reflective film having a size of, for example, 132 mm×132 mm, because the number of detected defects exceeds 100,000. Overlooking critical defects in defect inspection would cause failures in the later mass production process of semiconductor devices, leading to wasteful labor and economical loss.